1. Field
The embodiment(s) discussed herein are directed to an optical device that controls an optical signal in optical communication.
2. Description of the Related Art
Generally, ultra-high-speed WDM (wavelength division multiplex) systems are required to support features such as broadband, low voltage and low insertion loss.
A Mach-Zehnder modulator using an electrooptic crystal substrate of LiNbO3, LiTaO3 or the like and having a waveguide is an optical modulator which has low insertion loss, low frequency chirping and is less wavelength-dependent, and is therefore advantageous in WDM systems.
FIG. 1 illustrates a configuration example of an optical modulator as an optical device. FIG. 1A is a top view of the optical modulator, FIG. 1B and FIG. 1C are cross-sectional views along line A-A′ and line B-B′ of FIG. 1A respectively.
The optical modulator is formed as follows. Using an electrooptic crystal substrate of LiNbO3 or LiTaO3 or the like, an optical waveguide 2 (2A, 2B) is formed by forming a metal film on a part of a crystal substrate 1 and making the metal film thermally diffuse or realizing proton exchanges or the like in benzoic acid after patterning. Electrodes (signal electrode 3A and grounding electrode 3B) are then provided in the vicinity of the optical waveguide 2 to form the optical modulator.
In such a case, an isolation film of SiO2 or the like is formed as a buffer layer 4 between the signal electrode 3A, grounding electrode 3B, and the electrooptic crystal substrate 1 to prevent light absorption by the electrodes.
Here, to drive the optical modulator at high speed, terminals of the signal electrode 3A and grounding electrode 3B are connected together via a resistor to form a traveling wave electrode and a microwave signal is applied thereto from the input side.
In this case, an electric field causes refractive indexes of the pair of waveguides 2A, 2B to vary as +Δna, −Δnb respectively, which causes a phase difference between optical signals propagating through the waveguides 2A and 2B to vary. In this way, signal light whose intensity is modulated with respect to an optical signal incident on the waveguide 2 (in) on the incidence side is output from the output waveguide 2 (out).
Furthermore, by changing the sectional shapes of the signal electrode 3A and grounding electrode 3B to control the effective refractive index of microwave and causing the velocity of light to match the velocity of microwave, it is possible to obtain wideband optical frequency response.
However, when the absolute values of electric fields applied to the pair of waveguides 2A, 2B are different, Δna<Δnb results, producing a phenomenon (chirping) in transition from ON-state to OFF-state that the wavelength of output light varies.
To solve this problem, a crystal substrate, in a part of which a polarization-inverted region is formed, is used. That is, in FIG. 1, the portion surrounded by the broken line is a polarization-inverted region 5 and the signal electrode 3A is made to pass over the waveguide 2A in the non-inverted region outside the polarization-inverted region 5 (FIG. 1B) and over the waveguide 2B inside the inverted region 5 (FIG. 1C). In the figure, the arrow “+Z” denotes the polarization direction.
In FIG. 1A, when the length of the waveguide is approximately L1=L2, the phase of light passing through the waveguides 2A and 2B changes by +Δθs, −Δθg in the non-inverted region and by +Δθg, −Δθin the inverted region respectively. Here, Δθs and Δθg denote the amounts of phase variation of light by the signal electrode 3A (signal) and grounding electrode 3B (ground) respectively.
Therefore, the phases of light passing through the waveguides 2A and 2B vary by +(Δθs+Δθg) and −(Δθs+Δθg) respectively at a Y-branch waveguide 2D on the output side, resulting in a phase modulation with the two values having the same absolute value yet inverted signs. This allows wavelength chirping to be reduced to 0.
As another optical device using polarization inversion, Japanese Patent Laid-Open No. 2005-274793 discusses a variable chirp modulator using demodulation in a polarization-inverted region, Japanese Patent Laid-Open No. 2005-284129 discusses a modulator having an optical modulator with flattened frequency response characteristics and Japanese Patent No. 3303346 discusses an example of a wavelength conversion element.
Here, in the optical function element whose electrooptic crystal substrate includes the polarization-inverted region, stress is generated in the boundary between the polarization-inverted region 5 and crystal substrate 1 when an ambient temperature changes. This stress generated causes a problem that the refractive index of the optical waveguide portion varies with the temperature.
With regard to a strain in such an inversion boundary, “Suppression of thermal drift in an ultra-high-speed LiNbO3 optical modulator” T. Shiraishi et al., LEOS2007, TuC2 discusses an optical modulator device using a domain wall strain.
Furthermore, in accordance with a temperature variation, a change of refractive index of an optical waveguide has various adverse influences on device characteristics. In the Mach-Zehnder interferometer type optical modulator illustrated in FIG. 1 in particular, a refractive index change caused by temperature results in an operation point variation and deteriorates modulation characteristics significantly. This phenomenon is called a “temperature drift phenomenon.”
On the other hand, Japanese Patent Laid-Open No. 8-166566 discusses a technique of widening bandwidths of an optical modulator using a structure with long ridge-like grooves, that is, ridge grooves (ridge waveguides) formed on both sides of an electrooptic crystal substrate in which optical waveguides are formed.
Furthermore, Japanese Patent Laid-Open No. 2005-275121 discusses an optical modulator including both a ridge waveguide and the polarization-inverted region 5. In the example described in Japanese Patent Laid-Open No. 2005-275121, the ridge waveguide is applied to a region where the optical waveguide and microwave interact with each other.
FIG. 2 illustrates a typical optical modulator provided with the above described ridge waveguide and polarization-inverted region with a top view (A) and cross-sectional views (FIG. 2B: section along line A-A′) and (FIG. 2C: section along line B-B′). Parts similar to those in FIG. 1 are assigned the same reference numerals.
Here, a result of research conducted by the present inventor shows that in a case where there are ridge grooves 6 on a substrate surface between a domain wall 7 (boundary between a polarization-inverted region and non-polarization-inverted region) and waveguides 2A and 2B, stress generated from the domain wall 7 is reduced by the ridge grooves 6, which reduces the possibility of the stress affecting the waveguides 2A and 2B.
On the other hand, since the ridge waveguide having the ridge grooves 6 is formed in the vicinity of the waveguides 2A and 2B, light insertion loss thereof increases due to scattering by sidewall of the ridge structure. Therefore, optical modulators with no ridge waveguide formed are often adopted.
Moreover, even in the case where the ridge waveguide is formed, the formation of the ridge waveguide needs to be limited to a minimum area.
Furthermore, a result of research conducted by the present inventor proved that increasing the depth of the ridge grooves 6 increases the effect of reducing stress. However, increasing the depth of the ridge grooves 6 of a normal ridge waveguide too much results in an increase in the drive voltage, and therefore it is not possible to increase the depth for only the effect of reducing stress and the effect of reducing stress is insufficient. For example, Japanese Patent Laid-Open No. 8-166566 discusses that the depth of the grooves should be within a range of 1 to 10 μm.
It is therefore an object of the present invention to provide an optical device that solves problems associated with typical a optical device including the problem of a temperature drift phenomenon caused by such a polarization-inverted structure inside crystal and strain of an boundary of polarization inversion and sufficiently reduce the temperature drift without increasing light insertion loss nor deteriorating optical modulation characteristics of drive voltage or the like.